1. Field of the Invention
The present invention relates to superconducting oxide materials and processes for preparing the same. More particularly, it relates to thallium-containing superconducting oxide materials having high superconducting critical temperature (Tc) and higher superconducting critical current density (Jc) and processes for preparing the same.
2. Description of the Related Art
Discovery of new oxide type superconductor by Bednorz and Muller revealed the possibility of high temperature superconductors (Z. Phys. B64, 1986 p 189).
The new type oxide superconductor discovered by Bednorz and Muller is represented by [La, Sr].sub.2 CuO.sub.4 which is called the K.sub.2 NiF.sub.4 -type oxide having a crystal structure similar to known perovskite type oxides. The K.sub.2 NiF.sub.4 -type compound oxides show such higher Tc as 30 K. which are extremely higher than known superconducting materials.
C. W. Chu et al. reported another superconducting material of so-called YBCO type represented by YBa.sub.2 Cu.sub.3 O.sub.7-x having the critical temperature of about 90 K. (Physical Review letters, Vol. 58, No. 9, p 908). However, the critical temperature of this oxide superconductor is not so different from a boiling point 70 K. of liquid nitrogen and hence the other oxide materials which have much higher critical temperature, in other words, which have larger temperature margin have been demanded.
Maeda et al reported the other type new superconducting compound oxide of Bi--Sr--Ca--Cu--O system which show the critical temperature of more than 100 K. (Japanese Journal of Applied Physics. Vol. 27, No. 2, p 1209 to 1210).
Thallium type compound oxides are also high Tc superconductors of more than 100 K. The present inventors disclosed several kinds of thallium type compound oxides superconductors in U.S. Pat. application No. 223,634 filed on Jul. 25, 1988, now U.S. Pat. No. 4,880,773, and Hermann et al. reported Tl--Ba--Ca--Cu--O system in Appl. Phys. Lett. 52 (20) p 1738. U.S. Pat. No. 4,870,052 discloses a kind of thallium-containing oxide superconductor. Thallium type compound oxides have such a very important merit that superconductors which show such a high Tc as more than 100 K. can be obtained without using rear earth elements as a material so that the production cost can be reduced.
Although these known Tl--Ba--Ca--Cu--O system and Bi--Sr--Ca--Cu--O system oxide superconductors show very high Tco's at which the superconducting property is observed, their Tci's at which the apparent electrical resistance become zero or undetectable are such low as about 80 to 90 K. which is not so different from that of the Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x system because these compound oxide superconductors contain several different phases. Still more, their critical current density values (Jc) are inferior to that of the Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x system.
Still more, in the case of production of thallium type oxide superconductors, there is a special problem cause by such a fact that thallium is a very volatile element and toxic for human. In fact, it is difficult to obtain a thallium-containing oxide having a desired composition because the vapour pressure of thallium is relatively higher than the other elements. Further, the special attention must be paid to handle the material of the thallium-containing superconductor because thallium element is toxic for human.
An object of the present invention is to overcome the problems of the prior arts and to provide thallium-containing superconductors which is relatively easy to be obtained as a single phase and also which have improved superconducting properties and a method for producing the same.